It is well-known that hydrogen is a very high energy density element and clean-burning fuel. The energy density of hydrogen, which is around 120 MJ/kg, is more than double that of most conventional fuels, e.g., natural gas: 43 MJ/kg and gasoline 44.4 MJ/kg. Hydrogen can be combined with oxygen through combustion, or through fuel cell mediated oxidation/reduction reactions, to produce heat, or electrical power. The primary product of this reaction is water, which is non-polluting and can be recycled to regenerate hydrogen and oxygen.
Currently, hydrogen energetics is the focus of interest in nuclear industry, motor transport, auto industry, chemical industry, aerospace industry, portable power sources industry (cellular phones, computers, home appliances), etc. In particular, the transport sector is a consumer of about half of the world's crude oil production. Moreover, in large metropolitan agglomerations worldwide, road traffic represents one of the most important and fastest growing emission sources for both pollutants and noise. Hydrogen as a new vehicle fuel provides the opportunity for both, the reduction or avoidance of polluting emissions and the drastic reduction of the noise level produced. Already hydrogen operated internal combustion engines have a low noise potential and significantly reduced pollutant levels. Therefore, the transport sector of the economy is intensively adopting the use of hydrogen fuel. This can help solve environmental problems, especially in large megapolises and industrial regions.
One of the problems of hydrogen energetics is safe storage and delivery of hydrogen fuel to a combustion cell. Most generally, there are three basic hydrogen storage techniques. Hydrogen can be stored as a cryogenic liquid, as a compressed gas in a large vessel, or bound chemically in a compound such as a metal Hydride.
The infrastructural requirements for liquid hydrogen storage are high due to the very low cryogenic temperatures of −253° C. (20 K). Thus, liquid storage systems, transfer pipes and refueling couplings require significant thermal insulation in order to maintain the liquid state and avoid or retard premature rapid evaporation of liquid hydrogen.
Compressed hydrogen storage is the most common method for hydrogen storage. Typically the pressure levels are in the order of 20 MPa-70 MPa. Today's storage vessels usually are manufactured in fiber composite materials design in order to reduce structural weight. An internal shell is made of stainless steel or aluminum and is wrapped with glass and/or carbon fibers. The tank designs are also known which are made completely from plastic materials. Nevertheless, most compressed gaseous storage tanks are relatively large and heavy. Moreover, existing accumulation techniques with compressed gaseous hydrogen in tanks provide a relatively low hydrogen weight content (the ratio of the weight of hydrogen in accumulator to the weight of accumulator), i.e., less than 10 weight %, and there are certain restrictions for further growth of this parameter along with low explosion protection.
The storage of gaseous hydrogen in metal hydrides makes use of depositing hydrogen in metal alloys. The hydrogen accumulation and storage techniques are relatively explosion-proof, because hydrogen features no excess pressure. Disadvantages of metal hydride storage are that depending on the type of metal alloy, more or less elevated temperatures are needed to set hydrogen free again, and the low mass related storage density. Usually, the weight content of hydrogen is less than 4.5%.
Conceptually, also storage in other materials can be achieved by physical sorption. For example, storage of hydrogen in carbon nano-fibers is known. However, due to the weaker bonding of hydrogen in these solids the storage temperatures have to be lower than those for storage as metal hydrides.
It is known that hydrogen can be safely stored in micro-containers, such as hollow glass micro-spheres. The amount of hydrogen in each individual microsphere is very small, preventing the possibility of explosions by improper handling or during accidents.
If heated, the microsphere permeability to hydrogen will increase. Hydrogen can diffuse into the hollow cores of the micro-spheres through the thin glass walls at practical rates at temperatures between 100° C. and 400° C. This provides the ability to fill the micro-spheres with gas by placing the micro-spheres in high-temperature and high pressure environments. Once cooled, the micro-spheres lock the hydrogen inside since the diffusion rate is drastically lower at room temperature. A subsequent increase in temperature will increase the diffusion rate. Thus, the hydrogen trapped in the micro-spheres can be released by subsequently increasing the temperature.
For example, U.S. Pat. No. 4,328,768 describes a fuel storage and delivery system wherein hollow micro-spheres filled with hydrogen gas are stored in a fuel storage chamber at pressures of 400 atm. From the fuel storage chamber the micro-spheres are directed through a heated delivery chamber wherein hydrogen gas is freed by diffusion and delivered to an engine, after which the substantially emptied micro-spheres are delivered to a second storage chamber. The substantially emptied micro-spheres are removed by mechanical means, such as a pump, to a storage chamber from which they can be removed for refilling.